Many wellbore systems require downhole actuation of tools. Sliding sleeves are employed in apparatus for actuation of wellbore tools, wherein a plug structure, often called a ball, is launched to land in the sleeve and pressure can be employed to move the sleeve. Movement of the sleeve may open ports in the downhole tool, communicate tubing pressure to a hydraulically actuated mechanism, or effect a cycle in an indexing mechanism such as a counter. A sliding sleeve based wellbore actuator may be employed alone in a wellbore string or in groups. For example, some wellbore treatment strings, for example, those for introducing fluid along a length of a well, may include a number of sliding sleeve based wellbore actuators spaced apart. One wellbore treatment, know as wellbore stimulation, for example fracturing, employs a string with a plurality of sliding sleeve based wellbore actuators spaced therealong. The sliding sleeves are moveable to open ports through which wellbore treatment fluid can be introduced from the wellbore string to the wellbore to treat the formation. The sleeves can be opened in groups or one at a time, depending on the desired treatment to be effected.
Many sliding sleeve based actuators employ constrictions on the sleeve to catch the plug. The constriction protrudes into the inner diameter of the string and catches the plug when it attempts to pass. The constriction, or a sealing area adjacent thereto, creates a seal with the plug and forms a piston-like structure that permits a pressure differential to be developed relative to the ends of the sleeve and the sleeve is driven to the lower pressure side. The constriction on the sleeve may be a frustoconically tapering seat, dogs, collets, rings, etc. While some plugs actuate one sliding sleeve only, it is desirable sometimes to have a plug that actuates a plurality of sleeves as it moves through a string. Thus, some constrictions have been developed that are able to be overcome: to catch a plug, be actuated by the plug and then release it. Such constrictions may be deformable or convertible and therefore repeat-acting and the sleeves with which they are associated may be intended to be actuated more than once and/or may convert downhole.
While these sleeve based actuators have proven to be effective, some actuators have set diameters across their constrictions that limit the number of sleeves that can be employed in the well. On the other hand, while the deformable or convertible repeating ID constriction mechanisms allow greater numbers of sleeves, they can have complicated and sensitive mechanisms that can adversely impact cost and reliability.